In a color television receiver, an ACC circuit is usually employed in order to reproduce colors in a reliable manner. It is necessary to produce three color difference signals by a luminance signal and two signals obtained by demodulation of a chrominance signal. Since the chrominance signal is apt to be influenced by the condition of the propagation of the radio waves and the deterioration of the TV set, the amplitude of the chrominance signal may vary with respect to a predetermined value. Since the amplitude of a chrominance signal is usually different among a plurality of radio waves, a channel change is likely to produce a variation causing a TV set to reproduce unstable picutre colors. The ACC circuit contemplates eliminating the influence of the fluctuation of the chrominance signal for obtaining a stable color picture.
As well known, an ACC circuit for a TV receiver, includes a gain controllable amplifier, a burst-signal gate circuit, an amplitude comparator and an amplifier. The comparator is utilized for producing an output signal the magnitude of which is in proportion to the difference between the peak value of the burst signal and a predetermined value only when the peak to peak value of the burst signal is over a predetermined value.
In a conventional comparator, the burst signal is applied to a cathode of a diode via a capacitor wherein the cathode of the diode is connected to a base of a transistor the emitter of which is connected to ground and the anode of the diode is connected to ground. With this arrangement, when the peak to peak value of the burst signal is over a voltage expressed by V.sub.D +V.sub.BE, wherein V.sub.D is a forward voltage of the diode and V.sub.BE is a forward voltage across the base and the emitter of the transistor, the transistor becomes conductive. However, the values of V.sub.D and V.sub.BE of the diode and the transistor respectively have variations because of mass production. Therefore, the value of V.sub.D +V.sub.BE can not be set at a predetermined value. Moreover, the value of V.sub.D and V.sub.BE are apt to be influenced by the variation of the ambient temperature. Consequently, in a conventional type of comparator, the magnitude of the output signal of the comparator varies in accordance with the variation of the ambient temperature. Although the variation of the values V.sub.D and V.sub.BE among a plurality of comparators can be compensated for by adjusting the amplification degree of an amplifier responsive to the gain controllable amplifier, the variation due to the temperature variation is inherent to the conventional comparator.